How to Measure a Euro Cylinder: A Simple UK Guide to Getting the Right Size
Thursday, 12 November 2026
To measure a euro cylinder, measure from the centre of the fixing screw to each end of the cylinder, and record the two sides as internal × external, for example, 35 × 45 mm. Match both measurements to the replacement: don't choose a cylinder that is shorter or longer than the fit requires. Check the product page, as some manufacturers list the external measurement first.
Getting the size right matters whether you're replacing a cylinder in a UPVC, composite or timber door. This guide explains how to measure a euro cylinder in the door or after removing it, how to check the internal and external sides, and what to look for before ordering a replacement.
Why the right cylinder size matters
A cylinder that sticks out too far on the external side gives someone more to grip and attack. As a general guide, keep external projection to about 3 mm or less, following the cylinder and security furniture manufacturer's instructions. A correctly sized cylinder should sit flush with, or close to flush with, the handle or protective escutcheon.
A cylinder that is too short may sit too far inside the door furniture, making it difficult to operate or unsuitable for the lock. If the cylinder's cam does not align with the lock case, or the cylinder is forced during fitting, the mechanism may bind. Correct sizing helps the cylinder, cam and lock work together as intended.
What you'll need
- A steel ruler or tape measure marked in millimetres
- A note or phone to record the measurements
- A screwdriver, if you plan to remove the cylinder
- A phone camera to record which end faces outside
How to measure a euro cylinder in the door
- Open the door and keep it open while measuring. Find the long retaining screw in the door edge, level with the cylinder. The reference point is the centre of this screw, not the centre of the cam.
- Identify the two sides. Label them internal and external according to which way they face. A thumbturn is commonly on the inside, but check rather than relying on this convention.
- Measure the internal half. Measure from the centre of the fixing screw to the inside face of the handle or escutcheon where the cylinder should sit.
- Measure the external half in the same way, from the screw centre to the outside face of the door furniture.
- Write down both measurements clearly, such as "35 mm internal, 45 mm external". Check the product page to see how its size options are ordered before selecting one.
If the existing cylinder already sits proud of the furniture, don't copy that projection automatically. Check the door furniture depth and choose a cylinder that fits correctly without excessive external overhang.
How to measure a removed cylinder
If you can safely remove the cylinder, you can measure its two halves directly. Keep the door open, remove the retaining screw and turn the key slightly to align the cam with the cylinder body. The cylinder should slide out; don't force it. If you're unsure, ask a competent locksmith for help.
With the cylinder out, measure from the centre of the fixing screw hole to each end of the cylinder body. The screw hole divides the overall length into the internal and external halves. Record which end faced outside before removal, using a photo or a note.
The two measurements should add up to the cylinder's total length, which is a useful cross-check. But don't order based on total length alone: two cylinders can have the same overall length but different internal and external halves.
What to measure and why
| What to measure or check | How | Why it matters |
|---|
| Total length | Add the internal and external measurements, or measure end to end as a cross-check. | Confirms the overall length, but doesn't tell you how that length is divided. |
| Internal half | Measure from the fixing screw centre to the inside end or furniture face. | Helps the cylinder sit correctly on the inside of the door. |
| External half | Measure from the fixing screw centre to the outside end or furniture face. | Helps avoid excessive projection on the exposed side. |
| Cam type and alignment | Check that the replacement has a compatible euro cylinder cam and that it lines up with the lock case. | The cam operates the lock; an incompatible or misaligned one may prevent correct operation. |
| Fixing screw position | Use the centre of the retaining screw hole as your measurement reference. | Measuring from another point, such as the cam, can give you the wrong size. |
Measuring a thumbturn cylinder
The thumbturn itself does not add to the cylinder length. Measure the cylinder body from the fixing screw centre to each end, just as you would for a key-and-key cylinder. Then record which side has the thumbturn and make sure the replacement is listed for the same arrangement.
Choosing a replacement size
Euro cylinder sizes vary by manufacturer and product range. Options commonly come in steps of around 5 mm, but that isn't a definitive sizing list. Match your measurements to the available options and read the product page carefully: some manufacturers list internal × external, while others put external first.
If you're changing the cylinder to upgrade security, also consider the thickness of the door and the depth of the handles or security escutcheons. The cylinder should be long enough to operate correctly, but not unnecessarily long on the external side.
If you're replacing a standard cylinder, a TS007 3-star anti-snap cylinder is one of the best-value security upgrades to consider for a UPVC door. Anti-snap designs are intended to make it harder to use a snapping attack to reach the lock mechanism. Check the cylinder's marked external side and fit it the correct way round.
Browse our door cylinders or explore TS007 3-star and anti-snap cylinders. For further help measuring locks and hardware, visit our measurement guides.
Common measuring mistakes to avoid
- Measuring from the cam instead of the screw centre. Use the centre of the retaining screw or cylinder screw hole.
- Forgetting which side is which. Write "internal" and "external" beside your measurements, or take a photo before removal.
- Measuring with the key in. The key isn't part of the cylinder length.
- Ordering by total length only. Specify both halves, because their proportions matter.
- Assuming all euro cylinders are the same length. Sizes and available increments vary.
- Ignoring cam alignment. Confirm that the replacement cylinder's cam suits the lock case and lines up correctly.
- Assuming the first number is always internal. Check how that particular product page lists its sizes.
Need help getting the right part?
If you're unsure, our team can help identify a suitable replacement. Call 0330 174 0851, Monday to Friday, 9:30am–4:30pm, or email info@lockshopdirect.co.uk. You can send photos and your internal and external measurements.
Same-day dispatch is available on qualifying orders placed before 5pm, with next-day delivery on selected products. Delivery is free on orders over £60. Before ordering, write down both measurements with the internal and external sides labelled, then check the size convention on the product page.
Author: Penny
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